Field of the Invention
The present invention relates to a method for producing a wood-based board provided with a decor layer made of a decor paper.
Description of Related Art
When producing boards, in particular in form of laminated boards, typically different papers such as decor paper, overlays, backing papers, underlays and such are impregnated first in an impregnation line with a duroplastic resin, which contains usually water as solvent. The different papers are than subsequently dried to a defined humidity of about 5-7 wt %. Such completely impregnated papers are than pressed onto the wood-based boards in a press in a short cycle process for obtaining laminate boards.
Different methods are applied for the continuous processing of decor papers for producing the different products. While relative simple varnished finish films, i.e. glued papers with small amount of glue are used for furniture applications, papers or impregnates with higher amount of resin are used for flooring applications due to the higher mechanical stress when used, which guarantee a higher crack resistance. Thereby the continuous process has substantial advantages compared to the discontinuous process as for instance in the short cycle press regarding the processing. It is more efficient, there are less material losses due to breakage and the facilities are mechanically less elaborate.
An aggravating problem is that in the last years the demanded product quality poses higher demands to the manufacturing processes. This is valid for the complete value added chain in the production of laminate floors. In the impregnation process, wherein the different papers, which are used for the laminate floor production, are impregnated with a duroplastic resin, significant problems occur due to the swelling of the paper. This relates in particular to the decor papers. They change their dimensions due to water absorption in longitudinal and cross direction. If the decor layer is now subsequently pressed onto the wood-based board, the resin used as an impregnating means melts and connects the decor paper to the core of the board. It is however essential for the further processing of the wood based board provided with a complete impregnated decor paper layer that the swelling degree of the decor layer is considered in order to obtain a correlation of the decor to the format of the finished boards (length and width rapport) as well as possible. This is however very elaborate and often not possible due to the greatly varying swellings of the decor paper in length and width during the impregnating process.
In order to reduce or avoid said swelling problem of a complete impregnated decor paper layer the use of dry decor papers was suggested in the past. For example in DE 10 2007 026 170 A1 no impregnated or higher resinated paper is used, but rather a raw paper that is glued onto the carrier using a water containing glue. In EP 1 977 909 B1 it is also provided to place the dry decor paper onto a melamine resin layer applied to the upper side of a support board, to cover said paper subsequently with a melamine resin layer and to finally press said structure. Hereby the problem of paper swelling is minimized by using the dry paper.
The variants described at last have however problems during processing when contacting the aqueous resins due to swelling processes in the paper, which can provide wrinkle formation. In order to avoid this the guidance and fixation of the paper web has to be designed very exact. Furthermore, the process can cause swellings in the cover layer of the wood based board due to the significant amounts of water, which are applied onto the support by the glue. This causes a deterioration of the technological properties of the cover layer (cover layer surface soundness). This effect is the more severe the longer the water is exposed to the board before further refinement (system downtime, program change and so on).
The described variants seem to solve the problem of the paper growth by swelling on a first glance. However, a new problem arises, when decors are processed once by using dry decor paper webs and once according to the standard process using completely impregnated decor paper webs. As already described above the impregnated papers are swollen during the impregnating process and have thus a different length and width as the dried papers glued directly onto the board. If based thereon the same products are now produced in the same format again problems with the length and with rapport can occur.
Impregnated papers are typically pressed on a support by means of the short cycle press technology in a press. This short cycle technology has however also some disadvantages. Firstly, the impregnated paper sheets are relatively brittle and can be torn or destroyed when processing. Secondly, the production of the sheets is also susceptible for waist. Altogether said methods are also relatively expensive since a total of three papers are used.
Starting from this technological background a method shall be developed which reduces or eliminates said disadvantages of the above described methods.